The surfaces of downhole tools, when in contact with an abrasive environment such as a borehole wall, can undergo a high level of abrasion. In light of this, these surfaces are oftentimes coated with an abrasion resistant coating, in an effort to reduce wear and extend tool life. For example, abrasion resistant coatings, or hard facings, are often applied to susceptible areas of a tool such as wear bands, directional drilling pressure pads and stabilizers. Coatings such as these are typically a particulate metal matrix composite, based on a nickel or cobalt based alloy matrix containing tungsten carbide or titanium carbide particles. Using such a combination, both high degrees of hardness and toughness can be obtained.
These coatings are applied using a variety of methods such as weld overlays (MIG, plasma transfer arc, laser-cladding), thermal spray processes (high velocity oxygen fuel, D-gun, plasma spray, amorphous metal) and brazing (spray and fuse techniques) as known by those skilled in the art. In addition, wear resistant inserts, such as cemented tungsten carbide tiles or polycrystalline diamond (PDC, TCP) inserts are often attached to critical areas by brazing or other means to increase the wear resistance. Conventional abrasion resistant coatings such as these result in the application of a coating over a substrate that has a non-uniform surface that is oftentimes rough in texture.
While numerous hardfacing coatings have been produced for wear-resistant applications, none have been specifically designed to withstand the harsh environmental conditions encountered in downhole environments. The rubbing of a metal against a rock formation in the presence of drilling mud under high stress, together with repeated impact loading, creates a unique set of mechanisms that can lead to very rapid material loss.
In such an environment, the abrasive wear exhibited by traditional abrasion resistant coatings can be divided into two categories, namely brittle wear and ductile wear. Brittle wear occurs due to cracking and material removal at the surface of the abrasion resistant coating while ductile wear is exhibited by gradual material removal which results in a smoothing effect on the surface. The extent by which an abrasion resistant coating exhibits brittle or ductile wear is dependent on the local load the material must bear while in operation. For example, if the material at the surface of the abrasion resistant coating is brittle and the load applied is higher than its fracture stress (fracture under compressive load), the wear mechanism is brittle. In the alternative, if the load applied to the abrasion resistant coating is less than the fracture stress of the abrasion resistant coating, material is removed by a ductile wear mechanism. The wear rate under brittle wear is significantly higher than that in ductile wear. See I. M. Hutchings, Tribology: Friction and Wear of Engineering Materials, 1992 (incorporated herein by reference in its entirety).
Conventional approaches to minimizing wear in an abrasion resistant coating have resulted in the increase of the bulk hardness of the abrasion resistant coating by increasing the fraction of tungsten carbide reinforcement used in the abrasion resistant coating. Such an increase in the carbide volume fraction results in an increase of the wear resistance. However, at very high carbide volume fractions, extensive cracking can occur, as insufficient ductile matrix material is present to accommodate the residual stresses created during processing. For example, an abrasion resistant coating with a high carbide volume fraction applied using a plasma transfer arc method will likely result in a non-uniform surface that exhibits excessive cracking at various regions due to the lack of sufficient ductile matrix material.
Additionally, conventional methods for abrasion resistant coating leave a non-uniform surface finish exhibiting a rough texture with poorly attached clusters of solidified metal/carbide coatings. For example, during the aforementioned plasma transfer arc (PTA) technique, a powder is directed into a high temperature, ionized gas (i.e. plasma) that is created between a non-consumable electrode and a substrate. Temperatures in the plasma region range from 10,000-50,000 degrees F. (5,500-28,000 C.). Powder introduced into this region is melted and fusion welded to the underlying substrate. The fusion welded powder applied to the substrate has a rough surface finish and is non-uniform in nature, resulting in areas of weakly bonded globules of melted metal/carbide. When this surface is placed in contact with an abrasive environment, these weakly attached clusters of carbide readily detach from the surface of the abrasion resistant material, thereby causing accelerated wear and the formation of deep grooves which can nucleate and cause further surface damage to the abrasion resistant coating. In view of the above, a system, method and apparatus which results in the reduction of abrasive wear in abrasion resistant coatings is needed.